In any electric device, the effective power required for its proper operation is independent from the supply voltage. Thus, devices designed to operate under a 110-volt mains voltage are sized to withstand relatively high currents, while devices meant to be used under a 220-volt mains voltage are sized to withstand relatively high voltages. In other words, for a given operating power, the fact that a device is provided for, for example, two supply voltages (respectively 110 and 220 volts), imposes that its components have to withstand a current linked to the lowest voltage and a voltage linked to the highest voltage.
Therefore, for a device to be usable with any power supply, a simultaneous voltage and current oversizing must be provided.
In many applications, the A.C. mains voltage, whatever its value, has to be converted into a D.C. component. The characteristics of the converters used then depend on the supply.
FIG. 1 illustrates an example of conversion of an A.C. voltage Vin, of a given level chosen between several mains voltages, applied to the input terminals of a diode rectifying bridge 1. Conventionally, a load is connected between two output terminals A and G of the bridge. The case of a load to be supplied by a relatively stable voltage is considered hereafter. For example, the load includes the primary winding L1 of a switched-mode power supply (SMPS). To obtain a substantially D.C. load supply voltage, a capacitor Cout is connected, between output terminals A (positive) and G generally defining the ground) of bridge 1 and in parallel with the load to be supplied SMPS). Primary winding L1 of the SMPS supply is controlled by a MOS transistor MN, controlled by a signal which depends on the load applied to the secondary of the SMPS supply. This secondary is formed, for example, of a winding L2 across which are connected in series a diode DL and a circuit to be supplied (not shown). A capacitor CL is mounted in parallel with this circuit and in series with diode DL. The conventional operation of the secondary of the SMPS supply will not be discussed herein.
In such a structure shown in FIG. 1, if It is desired to be able to supply the converter under different voltages, capacitor Cout and transistor MN have to be able to withstand high voltages and high currents. A capacitor or a MOS transistor able to withstand a high voltage are expensive and bulky. Further, such a MOS transistor causes high energy dissipation.